update minimum supported python version to 3.8

Signed-off-by: Tomasz Kłoczko <kloczek@github.com>

setup.py

@@ -23,7 +23,7 @@ install_requires = [
     'pytz',
     # install requirements depending on python version
     # see https://www.python.org/dev/peps/pep-0508/#environment-markers
-    'backports.zoneinfo; python_version == "3.7" or python_version == "3.8"',
+    'backports.zoneinfo; python_version <= "3.8"',
     'tzdata'
 ]
 
@@ -39,7 +39,6 @@ setuptools.setup(
         'License :: OSI Approved :: BSD License',
         'Operating System :: OS Independent',
         'Programming Language :: Python',
-        'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: 3.10',
@@ -57,7 +56,7 @@ setuptools.setup(
     package_dir={'': 'src'},
     include_package_data=True,
     zip_safe=False,
-    python_requires=">=3.7",
+    python_requires=">=3.8",
     install_requires=install_requires,
     entry_points = {'console_scripts': ['icalendar = icalendar.cli:main']},
     extras_require={